Host-Guest Complexes between an Aromatic Molecular Tweezer and Symmetric and Unsymmetric Dendrimers with a 4,4'-Bipyridinium Core
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- 作者：Vincenzo Balzani ; Heinz Bandmann ; Paola Ceroni ; Carlo Giansante ; Uwe Hahn ; Frank-Gerrit Klä ; rner ; Ute Mü ; ller ; Walter M. Mü ; ller ; Carla Verhaelen ; Veronica Vicinelli ; and Fritz Vö ; gtle
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：January 18, 2006
- 年：2006
- 卷：128
- 期：2
- 页码：637 - 648
- 全文大小：467K
- 年卷期：v.128,no.2(January 18, 2006)
- ISSN：1520-5126
文摘
We have investigated the spectroscopic and electrochemical behavior of symmetric andunsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4'-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Fréchet-type) dendrons.The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor-acceptor interactions that arealso evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4'-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising anaphthalene and four benzene components bridged by four methylene units. Host-guest formation causesthe quenching of the tweezer fluorescence. The association constants, as measured from fluorescenceand 1H NMR titration plots, (i) are of the order of 104 M-1, (ii) decrease on increasing dendrimer generation,and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation.The analysis of the complexation-induced shifts of the temperature-dependent 1H NMR signals of the hostand guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allowsthe conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (
G
< 10kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentiallycomplexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping forthe symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causesa displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values,whereas the second reduction wave is unaffected. These results show that the host-guest complexesbetween the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and canbe reversibly assembled/disassembled by electrochemical stimulation.
G < 10kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentiallycomplexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping forthe symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causesa displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values,whereas the second reduction wave is unaffected. These results show that the host-guest complexesbetween the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and canbe reversibly assembled/disassembled by electrochemical stimulation.